The invention is in the field of electronic reproduction technology and is directed to a method for pixel-by-pixel and line-by-line opto-electronic scanning of screened image masters.
In electronic reproduction technology, image values are acquired and further-processed in a scanner device, also referred to as a scanner, by pixel-by-pixel and line-by-line scanning of an image master to be reproduced with an opto-electronic scanner element. The scanner element is essentially composed of a light source for the illumination of the image master to be scanned and of an opto-electronic transducer. The scanner can be designed as a flat bed scanner or as a drum scanner.
In practice, screened image masters must often be scanned. The raster of an image master to be scanned is defined by the screen frequency (screen dots/cm) and by the screen angle, whereas the scan fineness in the scanning of the image master is determined by the scan frequency (pixels/cm), whereby screen frequency and scanning frequency are spatial frequencies.
Beat frequencies as sums and differences between the sampling frequency and the harmonics of the screen frequency arise when scanning a screened image master. Low beat frequencies cause a visible Moirxc3xa9 that a viewer considers disturbing and, thus, diminishes the reproduction quality.
Patent Abstracts of Japan, Vol. 096, No. 006, Jun. 28, 1996, and JP 08 051536 A already disclose a method for avoiding moirxc3xa9 in scanning masters that arises due to image enlargement. In this known method, the image values are subjected to a Fourier transformation for the recognition of moirxc3xa9, and the moirxc3xa9 is eliminated by a following spatial filtering.
Patent Abstracts of Japan, Vol. 097, No. 001, Jan. 31, 1997 and JP 08 242364 A disclose another method for avoiding moirxc3xa9 when scanning masters on the basis of a spatial filtering.
EP 0 511 754 A and EP 0 301 786 A recite methods for the descreening of screened masters by filtering.
It is therefore an object of the present invention to improve a method for scanning screened image masters such that disturbing Moirxc3xa9 phenomena are avoided or are at least greatly reduced.
According to the present invention, a method is provided for pixel-by-pixel and line-by-line opto-electronic scanning of a screen image master with a scanner device. Before the actual scanning of the master, at least one master area as a screen area containing a screen is identified in the screened image master. Image values of the identified screen area of the image master present in a locus domain are converted by a Fourier transformation into a frequency domain as a spatial frequency spectrum. Screen width and screen angle of the screen of the image raster are identified from the spatial frequency spectrum. Scan frequencies from the spatial frequency spectrum that are optimum for a Moirxc3xa9-free scanning are determined for the identified screen angle and the identified screen width of the screen of the image master. The optimum scan frequencies that determine the scan fineness are set at the scanner device. The screen image master is scanned with the set optimum scan frequencies for acquiring image values required for reproduction of the screened image master.
The invention is explained in greater detail with reference to FIGS. 1 and 2.